1. Field of the Invention
The present invention generally relates to electric-current sensors, such as for power lines, and more particularly to optical fiber coils which, in response to the Faraday effect, sense with either interferometric or polarimetric techniques the magnetic field associated with an electric current.
2. Description of the Prior Art
Optical fibers which are sensitive to magnetic fields are known in the art, and are increasingly being used as optical current transducers (OCT's) for electric power utilities. A typical OCT uses a single-mode fiber formed into a coil surrounding the electrical conductor. The polarization of any light traversing the fiber will shift, in response to the change in any current flowing through the conductor, as a result of the magneto-optic "Faraday" effect. Further discussion of field-sensitive optical fibers is provided in U.S. Pat. No. 5,051,577 assigned to Minnesota Mining and Manufacturing Co. (3M--assignee of the present invention).
An optical medium becomes less suitable for use in a Faraday-effect sensor as its linear birefringence increases, which makes the coil more sensitive to external magnetic fields and distorts the polarized light signal. Care is particularly required in fiber selection since the formation of a coil from loops of a fiber introduces physical stresses which may further increase birefringence. Conventional techniques for minimizing linear birefringence have had limited success inasmuch as they have only reduced birefringent effects to a low level but not eliminated them completely. One method for reducing linear birefringence consists of twisting the fiber after it has been completely drawn from the preform ("twisted fiber"). Twisting effectively averages the fast and slow birefringent axes, yielding lower linear birefringence, but not completely eliminating it. Several authors even suggest maximizing circular birefringence so as to drown out linear birefringence (see U.S. Pat. Nos. 4,255,018 and 4,949,038, U.K. Patent Application No. 2,104,213, and Proc. SPIE 985 pp. 138-150), but this is actually a significant disadvantage since the circular birefringence can vary widely over time and affect the sensor's accuracy, requiring more frequent calibration. The circular birefringence is generally also temperature dependent, and a temperature change can thus result in a large shift in the orientation of the output polarization of the sensing coil.
A second method involves simultaneously spinning [and drawing the fiber from] the heated preform while drawing the fiber ("spun fiber"). A ratio of spin pitch to unspun-fiber beat-length can be selected which lowers the linear birefringence and also imparts an insensitivity to external stress. This approach usually, however, greatly increases the circular birefringence (as well as induced linear birefringence from coiling) which may be highly temperature dependent as shown in J. of Lightwave Tech. vol. 9, no. 8, pp. 1031-1037 (FIG. 11). Temperature sensitivity is also the subject of U.S. Pat. No. 4,563,639.
The third conventional method of reducing linear birefringence relies upon relieving or eliminating internal stresses present in the fiber coil. Such stresses may be produced by bending forces or transverse pressure applied to the fiber during manufacture, as well as from the stress induced when the coil is formed. Improvement in the performance of field-sensing optical fiber coils is consequently possible by annealing the coils at a temperature at which stress relaxation occurs. Linear and circular birefringence can be lowered, but not eliminated.
Optical fibers and coils of reduced linear birefringence, produced by any of the above methods, still exhibit undesirable characteristics. For example, it is not possible to remove linear birefringence due to geometric asymmetry by annealing a coil. Fibers which are spun while drawing, such as that described in U.K. Patent Application No. 2,101,762, exhibit induced linear birefringence after being coiled. Also, twisted fibers have a spin pitch which is limited by the fiber's fracture strength. It would, therefore, be desirable and advantageous to overcome these problems with a field-sensitive optical fiber coil wherein both linear and circular birefringence are negligible, and remain so over a wide range of temperatures, so as to produce a Faraday-effect sensor which operates with optimum sensitivity.